text: A circuit of fluid in the loop of Henle—an important part of the kidneys—allows for gradual buildup of the concentration of urine in the kidneys, by using active transport on the exiting nephrons (tubules carrying liquid in the process of gradually concentrating the urea). The active transport pumps need only to overcome a constant and low gradient of concentration, because of the countercurrent multiplier mechanism.
  Various substances are passed from the liquid entering the nephrons until exiting the loop (See the nephron flow diagram). The sequence of flow is as follows:
  * Renal corpuscle: Liquid enters the nephron system at the Bowman's capsule.
  * Proximal convoluted tubule: It then may reabsorb urea in the thick descending limb. Water is removed from the nephrons by osmosis (and glucose and other ions are pumped out with active transport), gradually raising the concentration in the nephrons.
  * Loop of Henle Descending: The liquid passes from the thin descending limb to the thick ascending limb. Water is constantly released via osmosis. Gradually there is a buildup of osmotic concentration, until 1200 mOsm is reached at the loop tip, but the difference across the membrane is kept small and constant.
  :For example, the liquid at one section inside the thin descending limb is at 400 mOsm while outside it is 401. Further down the descending limb, the inside concentration is 500 while outside it is 501, so a constant difference of 1 mOsm is kept all across the membrane, although the concentration inside and outside are gradually increasing.
  * Loop of Henle Ascending: after the tip (or bend) of the loop, the liquid flows in the thin ascending limb. Salt–sodium Na and chloride Cl ions are pumped out of the liquid gradually lowering the concentration in the exiting liquid, but, using the countercurrent multiplier mechanism, always pumping against a constant and small osmotic difference.
  :For example, the pumps at a section close to the bend, pump out from 1000 mOsm inside the ascending limb to 1200 mOsm outside it, with a 200 mOsm across. Pumps further up the thin ascending limb, pump out from 400 mOsm into liquid at 600 mOsm, so again the difference is retained at 200 mOsm from the inside to the outside, while the concentration both inside and outside are gradually decreasing as the liquid flow advances.
  :The liquid finally reaches a low concentration of 100 mOsm when leaving the thin ascending limb and passing through the thick one
  * Distal convoluted tubule: Once leaving the loop of Henle the thick ascending limb can optionally reabsorb and re increase the concentration in the nephrons.
  * Collecting duct: The collecting duct receives liquid between 100 mOsm if no re-absorption is done, to 300 or above if re-absorption was used. The collecting duct may continue raising the concentration if required, by gradually pumping out the same ions as the Distal convoluted tubule, using the same gradient as the ascending limbs in the loop of Henle, and reaching the same concentration.
  * Ureter: The liquid urine leaves to the Ureter.
  * Same principle is used in hemodialysis within artificial kidney machines.